1. Field of the Invention
The present invention relates to a method and an apparatus for grinding with electrolytic dressing, and more particularly to a method and an apparatus for grinding accompanied by electrolytic dressing of a metal bonded grinding wheel carried out along with in-process measurement of the dimensions of the grinding wheel.
2. Description of the Related Art
Japanese Laid-open Patent Publication No. 188266/1989 (Japanese Patent Application No. 12305/1988) filed by the same applicant as that of the present application discloses a method and an apparatus for electrolytically dressing a conductive grinding wheel. The conductive grinding wheel may be a metal bonded grinding wheel, for example, a cast iron fiber bonded diamond wheel, and the wheel is dressed by applying a voltage to the grinding wheel. This method and apparatus have been successfully applied to the mirror surface grinding of semiconductor material such as silicon wafers. In addition, the inventor of the present invention has developed a technique called "ELID grinding" (Electrolytic In-process Dressing) which was reported at a symposium held by The Institute of Physical and Chemical Research (RIKEN) of Saitamaken, Japan ("Recent trends in mirror surface grinding technology", May 5, 1991).
In the ELID grinding method, a workpiece is ground by applying a voltage between a conductive grinding wheel and an electrode while supplying conductive fluid between the wheel and the electrode. The wheel is then electrolytically dressed. The ELID apparatus comprises a conductive grinding wheel having a contact surface for contacting the workpiece, an electrode opposed to the grinding wheel and spaced a distance therefrom, a nozzle for supplying conductive fluid between the grinding wheel and the electrode, and a device (i.e., a power source and feeder) for applying a voltage between the grinding wheel and the electrode.
FIG. 7 (PRIOR ART) shows the mechanism of electrolytic dressing according to the ELID grinding method. During pre-dressing (See Portion (A) of FIG. 7), when grains protrude from the wheel, the electrical resistance between the wheel and the electrode is low so that the electric current between the wheel and the electrode is relatively high (5-10 A). Therefore, the bond material on the surface of the wheel is dissolved electrolytically, and the non-conductive diamond grains are exposed. After a number of grains have been exposed (Portion (B) of FIG. 7), an insulating or non-conductive film comprising iron oxide (Fe203) is formed on the surface of the grinding wheel so that the electric resistance of the wheel is then increased. As a consequence of the film formation, both the electric current and the dissolution of the bond material decrease, and the exposure of the grains is virtually completed. Under the condition shown in Portion (B) of FIG. 7, grinding with the wheel is started. As a result of grinding, insulating film and diamond grains are scraped off and removed while the workpiece is ground by the grinding wheel (Portion C of FIG. 7). When the grinding is continued (Portion (D) of FIG. 7), the insulating film is worn off the surface of the grinding wheel so that the electrical resistance of the wheel decreases and the electric current between the grinding wheel and the electrode increases. The dissolution of bond material thereafter increases, and the exposure of the grains is started again.
As mentioned above, during ELID grinding, the formation and removal of the insulation film occurs as shown in Portions (B) through (D) of FIG. 7, the dissolution of the bond material is regulated automatically and the exposure of the grains is also automatically controlled. The process shown in Portions (B) through (D) of FIG. 7 is hereinafter referred to as the "ELID cycle".
In the above-mentioned ELID grinding, since the grains are automatically exposed by the ELID cycle, choking of the wheel does not occur even if the grains are very fine. Thus, with ELID grinding an excellent ground surface having a mirror surface can be obtained by using very fine grains. Consequently, the ELID grinding method can maintain excellent grinding abrasiveness in a wide range of applications from high efficiency grinding to mirror surface grinding.
However, the nonconductive film formed on the surface of the grinding wheel in ELID grinding makes it difficult to exactly measure the dimensions of the grinding wheel. Accordingly, it is a problem that the change in the dimension of the grinding wheel with time requires much operator skill in grinding the workpiece to accurate dimensions and shapes.
In the ELID grinding of the prior art, since the formation and removal of the non-conductive film as well as the dissolution of the bond material of the grinding wheel are automatically carried out in the ELID cycle, the change in the dimension of the grinding wheel over time does not necessarily occur at a constant rate. Accordingly, for example, in grinding optical lenses with high accuracy, it is necessary to carry out the grinding by empirically anticipating the dimensional change of the grinding wheel by repeatedly interrupting the grinding operation and also repeatedly measuring the dimensions of the grinding wheel using a micrometer or the like. This requires much labor and a relatively high degree of operator skill and lowers the setup efficiency. It has therefore been desired to provide an in-process means which can measure the dimensions of the grinding wheel during the grinding operation.
In an attempt to meet the above demand, a non-contact method of measurement of the dimensions of the grinding wheel using various means such as laser or a capacitance-type sensors has been proposed and used in certain applications. However, a problem with these means is that the accurate measurement of the dimensions of the grinding wheel is interfered with by the grinding fluid which is often adhered to the surface of the grinding wheel during the ELID grinding operation. In addition, the accurate measurement of the dimensions of the bond portion of the grinding wheel, which actually performs the grinding, is interfered with by the nonconductive film formed on the surface of the grinding wheel during the grinding operation.
The present invention intends to solve the problems mentioned above. That is, it is an object of the present invention to provide a method and an apparatus for grinding with electrolytic dressing which can measure the dimensions of the grinding wheel during the grinding operation without being influenced by the grinding fluid or the nonconductive film and therefore can efficiently carry out a highly accurate grinding operation without a high degree of operator skill.